Visiting the US from Australia - Car insurance/rego etc

Hmm..nothings easy is it.

I have had a reply from one agency, South Coast Insurance and they have told me that the main issue is not having a physical address.. but with this, I can get insurance.. pity I have no family in the US :\. Looked into rental cars, there is an added fee per day for under 21s, and even under 25s.. Also, they do not insure under 21s at all, and at rental prices up to $1000-$1500/month i doubt renting a car is cheaper than puchasing in my situation.

I have been told by other Aussies that they have just bought a car with rego, and if pulled over found that a receipt for car/title along with a Aussie drivers licence it has gotten them along way.

I guess this is an option also, but would cancel out getting car insurance all together...Not really an option I want to pursue, but it seems to be one of the only ones at this stage.